The invention relates to glass fiber facing sheets for cement boards and a method of making same, and more particularly, relates to nonwoven, open mesh facing sheets formed of continuous glass yarns, to a method of making same and to engineered surfaces, specifically, cement boards including such facing sheet material.
Interior and exterior construction boards with cores of plaster, cement, or hybrid materials, such as cement boards or gypsum boards, are used in a wide variety of indoor and outdoor structural applications. For example, cement boards are used as a support surface for overlying materials such as wood siding, stucco, synthetic stucco, aluminum, brick, tile, stone aggregate and marble. In addition, cement boards are used in exterior insulating systems, commercial roof deck systems, and exterior curtain walls.
Generally, cement boards contain a core formed of a cementitious material which is interposed between two layers of facing material. The facing material advantageously contributes flexural and impact strength to the high compressive strength but brittle material forming the hardened cementitious core. In addition, the facing material can provide a curable surface and/or other desirable surface properties to the cement board.
One material which has been used to form facing sheet material is glass fibers. In addition to increased dimensional stability in the presence of moisture, glass fiber facings provide greater physical and mechanical properties to the cement board. As described, for example, in U.S. Pat. No. 5,371,989 to Lehnert et al., the method of making cement boards containing glass fiber facing sheets typically consists of providing a continuous feed of the facing material and depositing a cementitious slurry onto the top surface of the facing material. A second continuous feed of facing material is then applied to the top surface of the slurry. The slurry is then dried to harden the cementitious composition and to integrate the facing material in the cement board. The cement board is cut to a predetermined length for shipping and for its eventual use.
U.S. Pat. No. 4,647,496 to Lehnert et al. describes the use of randomly oriented fibrous glass mats as the facing material for gypsum boards. Additionally, U.S. Pat. No. 5,397,631 to Lehnert et al. describes coating a gypsum board including a randomly oriented glass mat with a water-resistant resinous binder to prevent the deterioration of the bond between the gypsum and the glass mat.
An alternative to the randomly oriented fibrous glass mats to reinforce cement boards is the use of open mesh glass scrims formed from continuous glass yarns. Because these scrims contain continuous yarns, they possess greater tensile strength than the randomly oriented fibrous glass mats. For example, U.S. Pat. No. 3,993,882 to Knauf et al. describes using a facing sheet formed of a glass fabric. In addition, published European patent application 637,658 to Bay Mills Limited describes the use of glass rovings to form nonwoven scrims for stucco boards.
In order to reduce glass usage, the mesh size of the glass scrims may be increased thus reducing the number of yarns or xe2x80x9cpicks per inchxe2x80x9d in the transverse and/or the longitudinal direction without reducing the strength of the cement board. Nevertheless, the resulting increase in the size of the mesh openings in the glass fiber facing sheet causes the other problems. Particularly, when the open mesh facing is applied to a cementitious slurry, pitting or the formation of indentations may occur in the center of the mesh openings which negatively affects the aesthetic appearance and surface properties of the cement board.
The present invention provides a glass fiber facing sheet for engineered surfaces such as for masonry applications, roofing applications, and the like, preferably for cement board. The glass fiber facing sheet provides a smooth surface which is essentially free of pitting. The glass fiber facing sheet is created from a minimal amount of material compared to glass fiber facing sheets that use a heavier weight scrim, thus reducing the cost and weight of the resulting cement board or other product but without significant negative impact on the physical and mechanical properties of the final product.
In accordance with the present invention, the glass fiber facing sheet comprises an open mesh glass scrim having a plurality of intersecting glass yarns which are bonded at their crossover points to provide dimensional stability to the scrim. The glass scrim of the present invention is preferably a nonwoven scrim comprising generally transverse and generally longitudinal yarns having less than 100 mesh openings per square inch, i.e., less than 10 by 10 picks per inch. Preferably, the glass yarns are bonded through the use of a polymeric binder. A polymer web is joined to one or both faces of the glass scrim and covers at least a portion of the mesh openings in the scrim. The polymer web is formed of melt blown polymer fibers which are preferably formed directly on the surface of the scrim. The melt blown polymer web is generally a low strength web that does not contribute significantly to the strength of the cement board. Nevertheless, the polymer web causes a cementitious slurry or other liquid used to form cement board, roofing materials and the like, to window pane evenly over the mesh openings on the exterior of the scrim thus improving the appearance and, at least in some cases the strength of the final product, e.g., cement board, by improving the interaction of the liquid or slurry and the glass scrim. Because the melt blown web is typically thin and is normally applied to only one face of the glass scrim, the opposed face of the scrim provides an exposed three-dimensional grid profile surface which remains available to interact mechanically with hardenable liquids such as the cementitious slurry used in the cement board. Accordingly, when used in forming cement board, the glass fiber facing sheet of the present invention is mechanically integrated into a surface portion of the cementitious core by virtue of the exposed grid profile surface of the scrim. A second open mesh glass scrim may be mechanically integrated into the other surface of the cement core to further enhance the properties of the cement board and may also include a melt blown polymer web on the surface of the scrim.
In one preferred embodiment of the invention, the melt blown polymer web applied to the surface of the glass scrim has a basis weight of between about 2 and 30 g/m2 (grams per square meters of scrim). Cement board formed using this facing sheet has a smooth exterior surface with little or no pitting. In another preferred embodiment of the invention, the melt blown polymer web applied to the surface of the open mesh glass scrim has a higher basis weight of between about 10 and 50 g/m2. The basis weight of the melt blown polymer web in this embodiment provides various desirable surface properties including a smooth finished exterior surface which can be painted directly.
In yet another preferred embodiment of the invention, the melt blown polymer web applied to the surface of the glass scrim has an even higher basis weight of between about 45 and 75 g/m2. The melt blown polymer web may then be subjected to heat and pressure to melt and coalesce the fibers in the polymer web to form a microporous layer. The microporous layer provides a water resistant surface which nonetheless allows gases such as water vapor to pass through the web. Alternatively, a microporous film such as a high density polyethylene film may be applied to the facing sheet prior to or following deposition of the melt blown polymer web to provide essentially the same result.
In a preferred method embodiment of the invention, the present invention includes a method of creating a glass fiber facing sheet for engineered surfaces on products such as cement boards. According to this preferred embodiment, the melt blown polymer web is generally formed directly on the surface of the scrim and is preferably formed from an adhesive polymer composition which adheres to the surface of the scrim. In another preferred method embodiment, the present invention includes a method of making an engineered surface comprising providing a method of making an engineered surface comprising providing a facing sheet comprising an open mesh glass scrim defined by a plurality of intersecting, continuous filament glass yarns bonded at crossover points thereof and a melt blown polymer web joined to one face of the glass scrim and covering at least a portion of the mesh openings thereon, the opposed face of the glass scrim defining an exposed grid profile surface, integrating the exposed grid profile surface of the facing sheet into a surface portion of a cementitious slurry layer, and allowing the cementitious slurry layer to harden to form the engineered surface.
The present invention further includes a method of making a cement board using the glass fiber facing sheet of the invention. The glass fiber facing sheets can be used to prepare cement boards of various types using conventional cement board manufacturing apparatus and manufacturing layouts. The method comprises providing a first facing sheet, preferably formed of glass fibers, and depositing a first cementitious slurry onto and through the facing sheet. A second cementitious slurry typically varying in material composition and/or viscosity from the first cementitious slurry may optionally be deposited on the first cementitious slurry. A glass fiber facing sheet formed according to the method described above is then applied to the cementitious slurry with the exposed grid profile directly contacting the cementitious slurry. An additional cementitious slurry preferably having a low viscosity may optionally be applied to the surface of the glass fiber facing sheet. The cementitious slurry(s) and the glass fiber facing sheet(s) are then preferably pressed to control thickness and consolidate the materials and the cementitious material hardened, for example by heating, to mechanically integrate the exposed three-dimensional grid profile surface of the glass fiber facing sheet into the cementitious core thereby forming the cement board.
Cement boards including the glass fiber facing sheet of the present invention have a smooth finish with little or no pitting. In addition, these boards can be lighter and/or less expensive than conventional cement boards while possessing mechanical and physical properties comparable to or exceeding conventional boards.